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United States Patent |
5,258,728
|
Taniyoshi
,   et al.
|
November 2, 1993
|
Antenna circuit for a multi-band antenna
Abstract
A branching filter, which is operatively connected between an antenna and a
communication device which utilizes different frequency bands, suppresses
a mutual interference between signals transmitted to and from the
communication device. An antenna circuit, which is operatively connected
between the antenna, or a branching filter, and the communication device,
converts an impedance with respect to a signal in a frequency band having
a lower frequency, and reduces loss resulting from a capacitive antenna
impedance.
Inventors:
|
Taniyoshi; Kiyoshi (Kobe, JP);
Kondo; Toshihiko (Kobe, JP);
Takayama; Kazuo (Kobe, JP)
|
Assignee:
|
Fujitsu Ten Limited (Hyogo, JP)
|
Appl. No.:
|
697624 |
Filed:
|
May 9, 1991 |
Foreign Application Priority Data
| Sep 30, 1987[JP] | 62-149952[U] |
| Sep 30, 1987[JP] | 62-149953[U]JPX |
Current U.S. Class: |
333/132; 333/32; 343/860; 370/339 |
Intern'l Class: |
H03H 007/446; H03H 007/38 |
Field of Search: |
333/126,129,132,32
343/715,858,860,862
455/78,82,83
370/36-38
|
References Cited
U.S. Patent Documents
1688036 | Oct., 1928 | Clement | 343/858.
|
2021734 | Nov., 1935 | Macalpine | 343/860.
|
2096782 | Oct., 1937 | Brown | 333/132.
|
3725942 | Apr., 1973 | Ukmar | 455/82.
|
3925729 | Dec., 1975 | Amoroso.
| |
4085405 | Apr., 1978 | Barlow | 333/129.
|
4095229 | Jun., 1978 | Elliot | 455/82.
|
4141016 | Feb., 1979 | Nelson | 455/83.
|
4268805 | May., 1981 | Tanner et al. | 333/129.
|
4527168 | Jul., 1985 | Edwards | 343/901.
|
4567487 | Jan., 1986 | Creaser, Jr. | 343/900.
|
4584587 | Apr., 1986 | Ireland | 343/745.
|
4658260 | Apr., 1987 | Myer | 343/792.
|
4660049 | Apr., 1987 | Shinkawa | 343/715.
|
4675687 | Jun., 1987 | Elliott | 343/715.
|
4721965 | Jan., 1988 | Elliott | 343/715.
|
4734703 | Mar., 1988 | Nakase et al. | 343/790.
|
4748450 | May., 1988 | Hines et al. | 343/820.
|
4829317 | May., 1989 | Shinkawa | 343/903.
|
4839660 | Jun., 1989 | Hadzoglou | 343/715.
|
4850034 | Jul., 1989 | Campbell | 343/715.
|
Foreign Patent Documents |
1075780 | Apr., 1980 | CA | 333/132.
|
2362889A1 | Dec., 1973 | DE.
| |
2538348A1 | Aug., 1975 | DE.
| |
2538348 | Mar., 1976 | DE.
| |
2755867C2 | Dec., 1977 | DE.
| |
24026 | Feb., 1977 | JP | 333/132.
|
54-58306 | May., 1979 | JP.
| |
149518 | Nov., 1980 | JP | 333/132.
|
61-227405 | Oct., 1986 | JP.
| |
62-173801 | Jul., 1987 | JP.
| |
62-179202 | Aug., 1987 | JP.
| |
1401524 | Jun., 1988 | SU | 333/132.
|
Other References
"Cellular Technology Promises More Channels", Danny Goodman, Technology
Today Feb. 1982, pp. 41-49.
|
Primary Examiner: Lee; Benny
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a Divisional application of Ser. No. 07/249,556, which
was filed on Sep. 26, 1988 and which issued on Dec. 10, 1991 as U.S. Pat.
No. 5,072,230.
Claims
We claim:
1. A branching filter which connects a signal line of a first communication
means, for transmitting and receiving at least in a first frequency band
f1, to a common multi-band antenna and which connects a signal line of a
second communication means, for receiving at least in a second frequency
band f2, to the common multiband antenna, wherein the first communication
means is a mobile telephone configured for transmission in a transmission
frequency band f1b and reception in a reception frequency band f1a, and
the second communication means is a radio set for receiving the frequency
band f2 which is lower than the frequency band f1 of the first
communication means, said branching filter comprising:
a band inhibiting means possessing an electrostatic capacity having an
increased impedance in the first frequency band f1 relative to an
impedance thereof in the second frequency band f2 and connected in series
between the signal line of the second communication means and the common
multi-band antenna, said band inhibiting means configured to inhibit a
transmission and reception signal in the frequency band f1 of the first
communication means;
said band inhibiting means including series connected first and second
resonance circuits, respectively coupled to the common multiband antenna
and the second communication means, for respectively resonating in the
reception frequency band f1a and the transmission frequency band f1b of
the first communication means, each of said first and second resonance
circuits comprising parallel connected capacitive and inductive elements.
2. A branching filter according to claim 1, further comprising a bypass
filter for passing the first frequency band f1 and blocking the second
frequency band f2, the bypass filter connected in series between the
signal line of the first communication means and the common multiband
antenna.
3. An antenna circuit provided between an antenna and an antenna input
circuit of a radio set for receiving a first radio signal in a first
frequency band f2a and a second radio signal in a second frequency band
f2b which is a higher frequency band than the first frequency band f2a,
said antenna circuit comprising:
a signal cable;
a first impedance conversion circuit connected between the signal cable and
the antenna for converting an impedance thereof in the first frequency
band f2a from a high impedance to a low impedance;
a first filter circuit connected in parallel to the first impedance
conversion circuit between the signal cable and the antenna for passing a
signal in the second frequency band f2b;
a second impedance conversion circuit connected between the signal cable
and the antenna input circuit for converting the impedance thereof in the
first frequency band f2a from a low impedance to a high impedance; and
a second filter circuit connected in parallel to the second impedance
conversion circuit between the signal cable and the antenna input circuit
for passing a signal in the second frequency band f2b;
wherein at least one of the first and second impedance conversion circuits
is comprised of a coil and a transformer having a primary and a secondary
winding, said coil connected in series between a corresponding one of the
first and second filter circuits and one of the primary and the secondary
winding of the transformer, said coil for reducing loss caused by a stray
capacity of the transformer.
4. An antenna circuit according to claim 3, wherein the first filter
circuit is a first series circuit of a coil and a capacitor connected in
series between the signal cable and the antenna, and the second filter
circuit is a second series circuit of a coil and a capacitor connected in
series between the signal cable and the antenna input circuit.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus employing a single antenna to
transmit and receive, at low loss and without mutual interference, signals
in different frequency bands, such as mobile telephone signals and radio
broadcasting.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional transmission/reception
apparatus 50 for a mobile telephone. For mounting a mobile telephone on an
automobile, the antenna provided for the reception of radio broadcasts is
shared because its transmission frequency band f1 is different from the
frequency band f2 of the radio broadcasts. In order to share the antenna
in this way, the signal line of the mobile telephone is connected with the
signal line of the radio set. Therefore, when a radio broadcast is
received while using the mobile telephone, the so-called beat noise is
mixed in the sound reproduced by the radio set. To prevent the generation
of such beat noise, the elements shown in FIG. 1 have been used hitherto.
The frequency band f2 of radio broadcasts is, in AM broadcasts, frequency
band f2a, that is, 500 to 1620 kHz, and, in FM broadcasts, frequency band
f2b, that is, 76 to 90 MHz. In the mobile telephone, on the other hand,
for radio communication with the ground station connected with the
telephone line, a frequency band f1a of 870 to 890 MHz is used in
receiving, and a frequency band f1b of 920 to 940 MHz is used in sending.
The prior art shown in FIG. 1 makes use of such a difference in frequency
bands.
In other words, as shown in FIG. 1, a radio set 51 is connected to an
antenna 53 by way of a low pass filter 52, and the mobile telephone 54 is
connected to the antenna 53 by way of a high pass filter 55. The signal
line connected to the mobile telephone 54 is joined to the signal line
connected to the radio set 51. During use of the mobile telephone 54,
since the frequency band f1 of the signals transmitted or received by the
mobile telephone 54 is relatively high, the radio set 51 will not generate
beat noise by the interference with the signal in the frequency band f2
used in the mobile telephone 54 owing to the low pass filter 52.
The equivalent circuit of the antenna 53 and the typical circuit
composition of the low pass filter 52 are shown in FIG. 2. A capacitor C11
is connected in series to a signal source 56, and coils L11 and L12 are
connected in series to this capacitor C11. The contact point 57 of coils
L11 and L12 is grounded by way of another capacitor C12.
The relation between voltage V11 generated in signal source 56 and output
voltage V12 of the low pass filter 52 due to electrostatic capacity of
capacitors C11 and C12 is as follows:
##EQU1##
That is, in the low pass filter 52, since the capacitor C12 is provided
between the signal line and the ground, the output voltage V12 of the low
pass filter 52 unfavorably becomes smaller than the generated voltage V11
in the signal source 56. In eq. 1, since radio broadcasts are to be
received, the attenuation of signals by coils L11, L12 is assumed to be
sufficiently small.
FIG. 3 is an equivalent circuit diagram in the frequency band f2a of AM
broadcast of an antenna 61 and a cable 62 in a different prior art device.
In a car-mounted radio set, it will be very convenient if FM radio signals,
AM radio signals, and mobile telephone signals can be received by one
antenna. In an antenna which is extended or retracted by a motor or the
like, a signal cable cannot be attached to the lower end of the antenna,
and it is difficult to shorten the signal cable. Accordingly, the cable
capacity of the signal cable increases, and the impedance derived from the
cable capacity becomes high. In particular, in radio signals of a
relatively low frequency band such as AM radio signals, the effect of
cable capacity becomes larger. Therefore, in a car-mounted antenna,
signals in a wide frequency band must be sent out to the radio set while
suppressing the loss by the signal cable.
The antenna 61 can be expressed in terms of antenna effective capacity Ce
and antenna reactive capacity Ca, and the AM radio signals received by
this antenna 61 can be expressed in terms of an alternating-current power
source V21. The cable 62 can be shown as a line l11 between terminals A1
and B1, and this line l11 is grounded by way of cable capacity Cb. The
signal at the terminal B1 is fed into a radio set. The voltage V22 at this
terminal B1 is expressed as follows:
##EQU2##
As expressed in eq. 2, supposing that the cable capacity Cb is large, the
gain of the AM radio signals of relatively low frequency received by the
antenna 61 is lowered so that the cable capacity Cb makes the receiving
sensitivity and the ratio of signal to noise (S/N ratio) drop.
To prevent such a drop in receiving sensitivity and S/N ratio, an amplifier
(not shown) is placed between the antenna 61 and the cable 62, that is, at
the position of terminal A1, so that the receiving sensitivity and S/N
ratio are improved. In such an antenna, since active elements are used,
they give rise to an increase in cost, and also involve other problems
such as maintaining a circuit characteristic of suppressing only the
distortion of signals at the time of input of a strong electric field. In
addition, new problems may be also experienced, such as loss due to
impedance conversion in the amplifier, and insufficient matching of
impedance.
SUMMARY OF THE INVENTION
It is hence a primary object of this invention to present a novel, improved
transmission and reception apparatus for automobiles which solves the
above-discussed problems.
It is another object of this invention to provide a branching filter
capable of suppressing the mutual interference of signals between plural
communication means using different frequency bands.
To achieve this object, a branching filter of this invention comprises:
a first communication means for transmitting at least in a first frequency
band f1;
a second communication means for receiving at least in a second frequency
band f2 which is different from the first frequency band f1; and
a band inhibiting means possessing an electrostatic capacity which has a
larger impedance in the first frequency band f1 and is connected in series
to the signal line of the second communication means.
The branching filter of this invention has the signal line from the
communication means for facilitating the transmission or reception of
signals at least in the first or second frequency band f1, f2 connected to
a common antenna.
The signal line of the second communication means is provided with band
inhibiting means having an electrostatic capacity in series with the
signal line and having a larger impedance in the first frequency band f1.
Therefore, electrostatic capacity does not occur between the signal line
of the second communication means and the ground, and the signal level
will not be reduced by the band inhibiting means. Besides, the signal in
the first frequency band f1 at least transmitted by the first
communication means is inhibited by the band inhibiting means, so that
there is no adverse effect on the reception of signals by the second
communication means.
Thus, according to this invention, the effect of the transmission signal of
the first communication means on the reception signal of the second
communication means can be suppressed without lowering the level of
reception by the second communication means, and mutual interference
between the transmission and reception signals of the antenna commonly
used in different frequency bands f1, f2 can be suppressed.
In a further different preferred embodiment, the band inhibiting means is a
parallel resonance circuit connected to the signal line, and its resonance
frequency is selected in the first frequency band f1.
In another preferred embodiment, the first communication means transmits
and receives signals for a mobile telephone, while the second
communication means is a radio set for receiving signals in the frequency
band f2 lower than the frequency band f1 of the first communication means,
and the band inhibiting means is designed to inhibit signal within the
transmission and reception frequency band f1 of the first communication
means.
In a further preferred embodiment, the band inhibiting means is a series
connection of parallel resonance circuits for resonating in the reception
frequency band f1a and the transmission frequency band f1b of the first
communication means.
In another preferred embodiment, a bypass filter for allowing signals in
the first frequency band f1 to pass and blocking signals in the second
frequency band f2 is provided in the signal line connecting the first
communication means and the antenna.
It is still a different object of the present invention to provide an
antenna circuit capable of enhancing the reception sensitivity and S/N
ratio in a wide frequency band.
To achieve the above object, in an antenna circuit according to the present
invention which is provided between the antenna and an antenna input
circuit of a radio set for receiving a first radio signal in a first
frequency band f2a and a second radio signal in a second frequency band
f2b which is a higher frequency band than the first frequency band f2a,
the improvement comprising:
a signal cable;
a first impedance conversion circuit connected between the signal cable and
the antenna for converting the impedance in the first frequency band f2a
from high impedance to low impedance;
a first filter circuit connected between the signal cable and the antenna
for allowing signals in the second frequency band f2b to pass;
a second impedance conversion circuit connected between the signal cable
and the antenna input circuit for converting the impedance in the first
frequency band f2a from low impedance to high impedance; and
a second filter circuit connected between the signal cable and the antenna
input circuit for allowing signals in the second frequency band f2b to
pass.
According to this invention, between the antenna and the signal cable is
disposed means for adjusting the impedance, said means being composed of a
first filter circuit for allowing the first radio signals in the first
frequency band f2a to pass, and a first impedance conversion circuit for
converting the impedance in the second frequency band f2b from high
impedance to low impedance. And between the signal cable and the antenna
input circuit of the radio set is disposed means for adjusting the
impedance, said means being composed of a second filter circuit for
allowing the second radio signals in the second frequency band f2b to
pass, and a second impedance conversion circuit for converting the
impedance in the first frequency band from low impedance to high
impedance.
The second radio signals are sent out to the radio from the antenna by way
of the first filter circuit, while the first radio signals are converted
with respect to impedance by the first impedance conversion circuit. Thus,
loss due to the cable capacity in the signal cable is reduced, and the
signal is transmitted to the radio set. The second radio signals are then
transmitted to the antenna input circuit radio set through the second
filter circuit, while the first radio signals are converted into an
impedance matched with the antenna input circuit of the radio set by the
second impedance conversion circuit, and are transmitted to the antenna
input circuit of the radio set. Therefore, the radio signals over a wide
frequency band can be transmitted to the radio set without increasing loss
in the antenna and signal cable.
In this way, according to this invention, when radio signals are received
by the antenna, the loss of reception signals due to capacitative
impedance of the signal cable may be reduced. Therefore, the reception
sensitivity and S/N ratio in a wide frequency band can be outstandingly
enhanced.
In a preferred embodiment, the first and second filter circuits are series
circuits of a coil and a capacitor.
In a preferred embodiment, the first and second impedance conversion
circuits are transformers.
In a still further preferred embodiment, at least one of the primary and
secondary windings of the transformer is connected in series with a coil
for reducing the loss due to the stray capacity of the transformer.
DESCRIPTION OF THE DRAWINGS
These and other objects of this invention, as well as the features and
advantages thereof, will be understood and appreciated more clearly from
the following detailed description in conjunction with the accompanying
drawings, in which:
FIG. 1 is a block diagram of a conventional transmission and reception
apparatus;
FIG. 2 is an electric circuit diagram showing the equivalent of an antenna
53 and a low pass filter 52 of a transmission and reception apparatus 50;
FIG. 3 is an equivalent circuit diagram in a frequency band of AM broadcast
in a conventional antenna 61 and a cable 62;
FIG. 4 is an overall schematic of a mobile transmission and reception
apparatus according to the present invention;
FIG. 5 is an electric circuit diagram of an embodiment of a branching
filter according to the present invention;
FIG. 6 is a graph showing frequency characteristics of a band inhibiting
filter 413;
FIG. 7 is a schematic of an embodiment of an antenna circuit according to
the present invention;
FIG. 8 is an equivalent circuit diagram of an antenna circuit for
explaining the principle of the present invention;
FIG. 9 is an equivalent circuit diagram for explaining the principle under
consideration with respect to the capacity Cf in the equivalent circuit
shown in FIG. 8;
FIG. 10 is a graph showing the relation between reception frequency f and
output voltage level V41 in the equivalent circuit shown in FIG. 9;
FIG. 11 is an equivalent circuit diagram in an AM radio signal frequency
band f2a of an antenna circuit; and
FIG. 12 is a schematic of still a further embodiment of an antenna circuit
according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, preferred embodiments of this invention are
described in detail below.
FIG. 4 is an overall schematic of a mobile transmission and reception
apparatus 101 according to the present invention.
On an automobile car body 102 is erected a multiband whip antenna 103 which
is used commonly for the transmission and reception of signals for a
mobile telephone and for the reception of a radio broadcasts. This antenna
103 is telescopically driven by a motor 104 installed at its lower end.
The antenna 103 is connected to a branching filter 106 by way of a coaxial
cable 105, and signals for the mobile telephone are transmitted or
received by a mobile telephone transmitter/receiver 108 by way of a
coaxial cable 107, while the reception signals of a radio broadcast are
transmitted to a radio set 111 by a coaxial cable 109 through an antenna
circuit 110.
FIG. 5 is an electric circuit diagram of a branching filter 106 in an
embodiment according to the invention. The antenna 103 mounted on an
automobile is connected to a band inhibiting filter 413 by way of a cable
105 which constitutes a signal line. The output of the band inhibiting
filter 413 is applied to a radio set 111 which constitutes second
communication means. The coaxial cable 105 is connected with a
transmitter/receiver 108 of a mobile telephone, which constitutes first
communication means, by way of a high pass filter 415.
The transmitter/receiver 108 of the mobile telephone performs radio
communications with the ground station connected in the telephone line
network in a first frequency band f1, that is, in a frequency band f1a of
870 to 890 MHz of received signals, and in a frequency band f1b of 920 to
940 MHz of transmitted signals. On the other hand, the radio broadcast
received in a radio set 111 using a second frequency band f2, that is, a
frequency band f2a of 500 to 1620 kHz for AM broadcasts, and a frequency
band f2b of 76 to 90 MHz for FM broadcasts. Therefore, during the
reception of a radio broadcast by radio set 111, if a mobile telephone is
used, it is sufficient for the signals in the frequency bands f1a and f1b
during reception and transmission to be inhibited by the band inhibiting
filter 413.
The high pass filter 415, operatively disposed between the coaxial cable
105 and the transmitter/receiver 108 of the mobile telephone, comprises a
series connection of capacitors C23 and C24. A connecting point 417 of
these capacitors C23 and C24 is grounded through a coil L23, thereby
allowing signals in the frequency band f1 of the mobile telephone to pass
thereby and cutting off the signals in the frequency band f2 of the radio
broadcasts. Meanwhile, the band inhibiting filter 413 is composed of a
first band inhibiting filter 418 for inhibiting the frequency band f1a of
870 to 890 MHz, and a second band inhibiting filter 419 for inhibiting the
frequency band f1b of 920 to 940 MHz.
The first and second band inhibiting filters 418 and 419 are connected in
series to the coaxial cable 105. The first band inhibiting filter 418
comprises coil L25 and a capacitor C25, while the second band inhibiting
filter 419 comprises coil L26 and capacitor C26. The inductance of coils
L25 and L26, and the electrostatic capacity of a capacitors C25 and C26
are properly selected so as to inhibit the signals in the above frequency
bands f1a and f1b.
FIG. 6 is a graph showing the frequency characteristics of the band
inhibiting filter 413. The band inhibiting filter 413 operates during the
use of the mobile telephone, and inhibits the transmission of signals the
transmission from the antenna 103 during a reception mode (i.e. frequency
band f1a), and the transmission of signals from the transmitter/receiver
108 of the mobile telephone during a transmission mode (i.e. frequency
band f1b). In the radio set 111, generation of noise does not matter if
such is at less than 110 dV .mu.v (+3 dBmW) at input voltage. On the other
hand, the transmission output of the transmitter/receiver 108 of the
mobile telephone is 5 W (+37 dBmW) in Japan. Therefore, the band
inhibiting filter 413 is composed so that the input signal level may be
attenuated more than 34 dB and delivered in the frequency bands f1a and
f1b of 870 to 890 MHz and 920 to 940 MHz. FIG. 6 shows the frequency
characteristics with respect to the input signal level VI.
Thus, in this embodiment, during use of the mobile telephone, interference
of reception signals (870 to 890 MHz) transmitted to the radio set 111 is
prevented by the first band inhibiting filter 418, whereas the
interference of transmission signals (920 to 940 MHz) transmitted to the
radio set 111 is prevented by the second band inhibiting filter 419. In
addition, between the signal line of the radio set 111 and the ground
there is no intervening electrostatic capacity such as that effected by a
capacitor so that a drop in voltage level induced by antenna 103 by band
inhibiting filter 413 during the reception mode of a radio broadcast will
never occur.
In this manner, without lowering the reception signal level of the radio
set 111, effects of the transmission and reception signals for the mobile
telephone on the reception of signals of a radio broadcast may be
suppressed, and mutual interference between the transmission and reception
signals of the antenna commonly used in different frequency bands f1 and
f2 may be suppressed.
FIG. 7 is a schematic of an antenna circuit 110 in a different embodiment
of this invention, and FIG. 8 is an equivalent circuit diagram associated
with AM radio frequency band f2a of an antenna circuit 501 for explaining
the principle of this invention. The antenna 500 is represented by an
antenna reactive capacity Ca connected to ground, and an antenna effective
capacity Ce connected in series with an AM radio signal which is a first
radio signal received by this antenna 500 is represented as an
alternating-current power source V31. A coaxial cable 109 is represented
by a line l61 between terminals B2 and P2, and this line l61 is grounded
by way of a cable capacity Cb. Between the antenna 500 and the coaxial
cable 109 is interposed a transformer 502 for changing the impedance of
the circuit. The signal at terminal P2 is transmitted to the antenna input
circuit in the radio set 111. The voltage V41 at this terminal P2 is
expressed as follows, denoting the ratio of the number of turns of the
coil at the input side to the output side of the transformer 502 n:1
##EQU3##
As understood from eq. 3, by additionally installing the transformer 502,
the effect relating to the cable capacity Cb may be reduced to 1/n.sup.2
of that in the circuit illustrated in FIG. 3. Therefore, the impedance
derived from the cable capacity Cb as taken at the terminal A2 is
converted to 1/n.sup.2 of that by the transformer 502 so that the loss at
the coaxial cable 109 may be reduced.
Referring to FIG. 7, the antenna circuit 110 is composed of an antenna 103,
the coaxial cable 109, an impedance adjusting circuit 513 interposed
between the antenna 103 and the coaxial cable 109, and the impedance
adjusting circuit 517 interposed between the coaxial cable 109 and the
radio set 111. In FIG. 4, meanwhile, the impedance adjusting circuit 513
is built in the branching filter 106. Reference numeral 104 denotes an
antenna motor, and 106 denotes a branching circuit.
The output from the antenna 103 is applied to the impedance adjusting
circuit 513 through the branching filter 106. The impedance adjusting
circuit 513 has a low impedance in the frequency band f2b of FM radio
signal, and comprises an FM radio signal filter circuit 514 which
constitutes a first filter circuit, and an impedance conversion circuit
515 which comprises a transformer 522 and constitutes a first impedance
conversion circuit connected in parallel to circuit 514. The FM radio
signals received by the antenna 103 are delivered to the coaxial cable 109
through FM radio signal filter circuit 514.
The FM radio signal filter circuit 514 is composed, for example, of a
series connection of a coil 520 and a capacitor 521, and functions as a
high pass filter with a low impedance against FM frequency band f2b.
The radio signal from the coaxial cable 109 is transmitted to the impedance
adjusting circuit 517. The impedance adjusting circuit 517 is composed of
an FM radio signal filter circuit 518 which filters FM radio signals and
constitutes a second filter circuit, and an impedance conversion circuit
519 which effects impedance conversion action on AM radio signals and
constitutes a second impedance conversion circuit.
The FM radio signal filter circuit 518 is connected in parallel to the
impedance conversion circuit 519, and the FM radio signals from the
coaxial cable 109 are led out into the antenna input circuit of the radio
set 111 through the FM radio signal filter circuit 518. The FM radio
signal filter circuit 518 is, for example, composed of a coil 523 and a
capacitor 524, and functions as a high pass filter for filtering
relatively high frequency signals such as FM radio signals. The impedance
conversion circuit 519 comprises a transformer 525 as in the first
impedance conversion circuit 522 mentioned above.
Therefore, the inductance of coils 520 and 523 in the FM radio signal
filter circuits 514 and 518, and the electrostatic capacity of capacitors
521 and 524 are properly selected so as to possess the resonance frequency
in the FM radio signal frequency band, respectively.
In the circuit 501 shown in FIG. 8, however, there is actually an effect of
the capacity in the FM radio signal filter circuit 514 shown in FIG. 7. An
equivalent circuit diagram which illustrates the principle under
consideration related to such a capacity component Cf is shown in the
circuit 501 of FIG. 9. Reference numerals A2, B2, P2 and 61 are the same
as those shown in FIG. 8. For the sake of simplicity, the antenna
effective capacity Ce and the antenna reactive capacity Ca are
collectively expressed as C.sub.A. Incidentally, the transformer 502
corresponds to the transformer 522 in FIG. 7, while the antenna 500
corresponds to the antenna 103. A self-inductance L.sub.1 is provided at
the input side, a self-inductance L.sub.2 is provided at the output side,
and there is a mutual inductance M between the input side and the output
side. Therefore, between the alternating-current power source V31 derived
from the radio signal received by the antenna 500, and the voltage level
V41 applied to the radio set 111, the following relation is established,
assuming the current from the antenna 500 to be i1, the current flowing in
the capacity component Cf to be i2, and the current due to cable capacity
Cb to be i3:
##EQU4##
Therefore, solving the above equations, the following relation is
established:
##EQU5##
Where .omega. denotes the angular frequency of the received radio signal.
At this time, when the denominator of eq. 8 is zero, V41 reaches the
maximal value. Supposing here that the mutual inductance M is expressed as
kL.sub.1 .multidot.L.sub.2 (where k is a coupling coefficiency of
transformer 502), the maximal value of V41 is expressed as follows:
##EQU6##
Thus, as shown in eq. 9 the voltage level V41 comes to possess the maximal
value with respect to two values differing in frequency f. Supposing the
frequencies corresponding to the maximal value of voltage level V41 to be
f11, f12 (f11<f12), the relation between frequency f and voltage level Vc
is shown in FIG. 10. As understood from eq. 9 to eq. 11, as the coupling
coefficient k becomes smaller, the frequency f12 becomes lower. Therefore,
by increasing the coupling coefficient k possessed by the transformer 502,
when the AM radio frequency band f2a is adjusted to settle within
frequency f11 and frequency f12, a flat reception characteristic will be
obtained in the AM radio signal frequency band f2a. A transformer 502
capable of increasing the coupling coefficient includes, for example, the
so-called sandwich winding or bifilar winding type.
FIG. 11 is an equivalent circuit diagram in an AM radio signal frequency
band f2a of the antenna circuit 110 in FIG. 7. The transformers 522 and
525 correspond to those shown in FIG. 7 and Cb denotes the cable capacity.
The antenna 103 may be represented as a capacity C.sub.A comprising the
antenna effective capacity possessing a series electrostatic capacity with
respect to the radio signal, and the antenna reactive capacity generated
between the radio signal and ground. Referring again to FIG. 7, the radio
signal received by antenna 103 may be represented by alternating-current
power source V32.
The AM radio signal received by antenna 103 has a high impedance in the FM
radio signal filter circuit 514, and therefore are led into the impedance
conversion circuit 515. In the impedance conversion circuit 515, the turn
ratio of the number of turns at the input side and the output side of the
transformer 522 is n:1. Accordingly, the voltage of the AM radio signal is
reduced to 1/n and the impedance is reduced to 1/n.sup.2 by the
transformer 522. The coaxial cable 109 gives rise to a cable capacity Cb
between the radio signal and ground.
Relative to a high frequency signal, for example, a FM radio signal, the
coaxial cable 109 has a low impedance. However, with respect to a
relatively low frequency signal such as an AM radio signal, the impedance
of the coaxial cable 109 due to cable capacity Cb is large. In this
embodiment, the impedance of the AM radio signal is reduced by the
impedance conversion circuit 515, so that the loss relating to cable
capacity Cb may be reduced.
The signal in a relatively low frequency band f2a such as an AM radio
signal from the coaxial cable 109 is high in impedance in the FM radio
signal filter circuit 518, and is led to the impedance conversion circuit
519. In the transformer 525 of the impedance conversion circuit 519, the
ratio m of the number of turns 1 at the input side to that at the output
side is set, and the AM radio signal led to this transformer 525 is
amplified in voltage, and is delivered into the antenna input circuit of
the radio set 111.
The relation between the alternating-current power source V32 and the
output voltage V42 is expressed in the following equation.
##EQU7##
A capacity C.sub.TA of the antenna circuit 110 as seen from the radio set
111 is expressed as follows:
##EQU8##
For example, this capacity C.sub.TA is defined at 80 pF in correspondence
with the impedance matching with the radio set, and the capacity C.sub.A
and the cable capacity Cb are determined by the length of the antenna 103
and the coaxial cable 109. Therefore, the turn ratios n and m of the
transformer 522 and 525 are selected so as to satisfy eq. 14 above.
The equivalent circuit of antenna circuit 110 as seen from the radio set
111 may be expressed as the inductance L.sub.0 /2 and capacity C.sub.TA
connected in parallel, assuming the inductance at transformers 522 and 526
to be L.sub.0. Supposing the resonance frequency of such a circuit to be
fp, the inductance L.sub.0 may be expressed as follows:
##EQU9##
It is desired to flatten the frequency characteristics in the AM radio
signal frequency band f2a by selecting the resonance frequency fp at, for
example, 250 kHz or other frequency outside the AM radio signal frequency
band f2a. Accordingly, the inductance L.sub.0 of the transformers 522 and
525 is determined by eq. 15.
Thus, in the antenna circuit 110, for example, when an AM radio signal and
a FM radio signal are commonly received by one antenna 103, the loss of
the AM radio signal at the coaxial cable 109 may be lowered. For instance,
assuming the antenna effective capacity Ce to be 15 pF, the antenna
reactive capacity Ca to be 5 pF, the cable capacity Cb to be 120 pF, and
the turn ratios n and m to be 4, the gain is improved by about 9 dB as
calculated according to eq. 2 and eq. 3.
In the foregoing embodiments, the loss will be greater if too large of a
value is set for the turn ratios n and m of the transformers 522 and 525,
or the effect will be smaller if too small of a value is used. According
to an experiment conducted by the present inventors, favorable results are
obtained when a numerical value of 10 or less is selected for the turn
ratios n and m.
FIG. 12 is a schematic of an antenna circuit 531 in still another
embodiment according to the present invention. The parts corresponding to
the foregoing antenna circuit 110 are identified with same reference
numbers. Reference numeral 104 denotes an antenna motor, and 106 denotes a
branching circuit. In the antenna circuit 531, the impedance conversion
circuit 515a of the impedance adjusting circuit 513a comprises coils 532
and 533 and the transformer 522. And, in the impedance adjusting circuit
517a, the impedance conversion circuit 519a comprises coils 534 and 535
and the transformer 525. In order to reduce the loss due to the stray
capacity associated with the transformers 522 and 525, coils 532 to 535
are employed at the input end and the output end of the transformers 522
and 525, respectively. As a result, the loss attributable to the stray
capacity of the transformers 522 and 525 is prevented, and the reception
sensitivity and the S/N ratio may be further enhanced.
In the foregoing embodiments, the loss in the AM radio signal frequency
band f2a due to stray capacity, in particular, can thus be reduced, while
the reception sensitivity and the S/N ratio in the radio receiver may be
outstandingly enhanced. Therefore, when receiving signals in a wide
frequency band by a signal antenna, for example, both FM and AM radio
signals are particularly effectively received by a car-mounted antenna
constructed according to the present invention.
Besides, depending on the type of antenna in general the antenna reactive
capacity varies more significantly than the antenna effective capacity.
When this invention is applied to an antenna with a large antenna reactive
capacity, its effect will be manifest. Meanwhile, the polarity of the
transformers 522 and 525 may be either normal phase or reverse phase, but
according to experiments, a greater effect will be obtained when
transformers 522 and 525 of a normal phase are used.
This embodiment is described with respect to receiving an FM radio signal
and an AM radio signal. However, it may be also favorably embodied in
applications in which radio signals and other signals such as mobile
telephone signals are received at the same time.
The invention may be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. The present
embodiments are therefore to be considered in all respects as illustrative
and not restrictive, the scope of the invention being indicated by the
appended claims rather than by the foregoing description and all changes
which come within the meaning and the range of equivalency of the claims
are therefore intended to be embraced thereby.
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